Something demanded or asserted; especially, a position or supposition assumed without proof, or one which is considered as self-evident; a truth to which assent may be demanded or challenged, without argument or evidence.

The enunciation of a self-evident problem, in distinction from an axiom, which is the enunciation of a self-evident theorem.

The distinction between a postulate and an axiom lies in this, -- that the latter is admitted to be self-evident, while the former may be agreed upon between two reasoners, and admitted by both, but not as proposition which it would be impossible to deny.
Eng. Cyc.